To enable acquisition of a base station by a mobile terminal included in a telecommunication system based upon the 3GPP standard TDD mode or the like, the corresponding receiver is to carry out the function of frame synchronization and identification of the so-called codegroup. Performing these functions is essential for the execution of the subsequent steps in the context of the cell-search system.
In particular, when a mobile terminal is turned on, it does not have any knowledge of the timing of the transmitting cell to which it is to be assigned. The 3GPP standard, therefore, proposes an initial cell-search procedure for acquiring the cell signal and synchronizing therewith.
The procedure basically comprises three steps, which in the TDD version are indicated as follows: Primary Synchronization Code Acquisition (first step); Codegroup Identification and Slot Synchronization (second step); and Downlink Scrambling Code, Basic Midamble Code Identification and Frame Synchronization (third step).
In the implementation of the second step described above it is assumed that the primary synchronization code, which indicates the position of the generic frame of the synchronization slot, has previously been acquired during the first step.
To obtain at this point the slot synchronization and identify the codegroup, to which the offset of the cell is associated, in the second step the Secondary Synchronization Channel (SSCH) is used. There are transmitted, in each synchronization slot, three secondary synchronization codes or words of 256 chips (i.e., letters), where the generic code is designated by Cm, with m ranging from 0 to 15.
In the known prior art approaches, for example from the international patent application No. WO 00/74276, execution of the second step of the cell search envisages that the secondary synchronization codes Cm, contained in the secondary synchronization channel (SSCH), will be extracted by a correlation process. The samples of the signal received are correlated with the possible secondary synchronization codes Cm transmitted on the SSCH. The set of three codes which presents the highest correlation energy is then identified, and the phases associated to the codes of the set of three are thus used for defining, according to the standard, the codegroup parameters and other parameters for frame synchronization, such as slot offset and frame number.
The above approach is schematically represented in the diagram of FIG. 1, where the reference number 10 designates a bank of twelve complex finite-impulse-response (FIR) filters, which are coupled to the twelve possible secondary synchronization codes SSC. The samples of the signal received r are sent at input to the bank 10 of complex FIR filters, and at the twelve outputs of the bank 10 there are generated signals indicating the correlation energies corresponding to the codes Cm, which are sent to a system for detection of the maximum value. The system is designated by 11.
The system for detection of the maximum value 11 determines the three codes Cm having the highest correlation energy, thus storing its relevant code Cm, its energy and its position or phase offset in the frame. The three codes Cm thus identified are sent to a comparison block designated by 12.
The block 12 performs an operation of comparison with a table which gives, according to the possible combinations of the phase offsets of the set of three codes Cm identified, phase offsets that are designated generically by bi, and can assume the values +1, −1, +j and −j, of the corresponding codegroups CD, slot offset or offset time toffset, i.e., temporal distance between start of a slot and start of the synchronization code, and frame_number FR (even or odd frame), which are then supplied at an output by the comparison block 12.
The approach according to the known art represented in FIG. 1 involves searching in the appropriate tables, which are stored. The tables enable, on the basis of the sets of three codes received on the SSCH, all the parameters of interest to be obtained. In the standard there are defined two possible cases of transmission of the SSCH.
In a first case, referred to as Case 1 in the 3GPP standard, the sequence associated to the SSCH is transmitted in just one slot for each frame. Illustrated therefore in TABLE 1 is the allocation table of codes Cm for the SSCH in the first case, hereinafter defined as a one-slot table.
TABLE 1CodegroupCodesetFrame 1Frame 2CGCSFR_1FR_2toffset 01C1C3C5C1C3−C5t0 11C1−C3C5C1−C3−C5t1 21−C1C3C5−C1C3−C5t2 31−C1−C3C5−C1−C3−C5t3 41jC1jC3C5jC1jC3−C5t4 51jC1−jC3C5jC1−jC3−C5t5 61−jC1jC3C5−jC1jC3−C5t6 71−jC1−jC3C5−jC1−jC3−C5t7 81jC1jC5C3jC1jC5−C3t8 91jC1−jC5C3jC1−jC5−C3t9101−jC1jC5C3−jC1jC5−C3t10111−jC1−jC5C3−jC1−jC5−C3t11121jC3jC5C1jC3jC5−C1t12131jC3−jC5C1jC3−jC5−C1t13141−jC3jC5C1−jC3jC5−C1t14151−jC3−jC5C1−jC3−jC5−C1t15162C10C13C14C10C13−C14t16172C10−C13C14C10−C13−C14t17...........................202jC10jC13C14jC10jC13−C14t20...........................242jC10jC14C13jC10jC14−C13t24...........................312−jC13−jC14C10−jC13−jC14−C10t31
The one-slot table illustrated in TABLE 1 comprises 6 columns corresponding to the codes, in which each element requires 6 bits: two bits for the phase and four bits for the code identifier. The column corresponding to the codegroup has 5-bit elements. The column for the offset time toffset has 12-bit elements, and the column for the codeset has a 1-bit element. The one-slot table therefore has a total size of 1728 bits.
In a second case, referred to as Case 2 in the 3GPP standard, the sequence is transmitted in two slots for each frame. The distance between the two slots are fixed at eight slots. Illustrated in TABLE 2 is the allocation table of the codes for the SSCH in the second case, hereinafter defined as a two-slot table.
TABLE 2Code-Code-groupsetFrame 1Frame 2CDCSSlot kSlot k + 8Slot kSlot k + 8toffset 01C1C3C5C1C3−C5−C1−C3C5−C1−C3−C5t0 11C1−C3C5C1−C3−C5−C1C3C5−C1C3−C5t1 21jC1jC3C5JC1jC3−C5−jC1−jC3C5−jC1−jC3−C5t2 31jC1−jC3C5JC1−jC3−C5−jC1jC3C5−jC1jC3−C5t3 41JC1jC5C3JC1jC5−C3−jC1−jC5C3−jC1−jC5−C3t4 51JC1−jC5C3JC1−jC5−C3−jC1jC5C3−jC1jC5−C3t5 61JC3jC5C1JC3jC5−C1−jC3−jC5C1−jC3−jC5−C1t6 71jC3−jC5C1JC3−jC5−C1−jC3jC5C1−jC3jC5−C1t7 82C10C13C14C10C13−C14−C10−C13C14−C10−C13−C14t8 92C10−C13C14C10−C13−C14−C10C13C14−C10C13−C14t9102jC10jC13C14jC10jC13−C14−jC10−jC13C14−jC10−jC13−C14t10112jC10−jC13C14jC10−jC13−C14−jC10jC13C14−jC10jC13−C14t11122jC10jC14C13jC10jC14−C13−jC10−jC14C13−jC10−jC14−C13t12132jC10−jC14C13jC10−jC14−C13−jC10jC14C13−jC10jC14−C13t13142jC13jC14C10jC13jC14−C10−jC13−jC14C10−jC13−jC14−C10t14152jC13−jC14C10jC13−jC14−C10−jC13jC14C10−jC13jC14−C10t15163C0C6C12C0C6−C12−C0−C6C12−C0−C6−C12t16.............................................233jC6−jC12C0JC6−jC12−C0−jC6jC12C0−jC6jC12−C0t20244C4C8C15C4C8−C15−C4−C8C15−C4−C8−C15t24.............................................314jC8−jC15C4JC8−jC15−C4−jC8jC15C4−jC8jC15−C4t31
The two-slot table illustrated in Table 2 comprises 12 columns corresponding to the codes in which each element requires 6 bits: two bits for the phase and four bits for the code identifier. The column corresponding to the codegroup CD has 5-bit elements. The column for the toffset has 12-bit elements, and the column for the codeset CS has 1-bit elements. The two-slot table therefore has a total size of 2912 bits.
An example of the second case is provided in FIG. 2, where there is a schematic representation of a frame TDD, which may, for example, be the frame FR_1, made up of a number of slots, including the two slots k and k+8. Also indicated in exploded form in FIG. 2 are the contents of the slot k+8, which comprises the primary channel indicated by its code Cp and the SSCH, made up of the codes Cm appearing in TABLES 1 and 2 and of the corresponding phases bi, which can assume the values +1, −1, +j, −j.
Likewise, for the third step of the cell-search procedure it is assumed that all the necessary information has received from the preceding step, including the codegroup CD.
On the basis of this information the signal received in the appropriate time window is correlated with the local replications of the four possible midamble codes, which come under the codegroup identified previously, by a search in an appropriate correspondence table. The scrambling codes SCR correspond to the cell are obtained.
The sequence or burst associated to the Primary Common Control Physical Channel (P-CCPCH), on which to perform the third step of the cell-search procedure, is transmitted concomitantly with the first SSCH of each frame. Shown in TABLE 3 is the allocation table for the third step of the cell search to be stored in the appropriate circuits.
TABLE 3Associated CodesLongCode-ScramblingBasicShort BasicCELLgroupCodeMidambleMidamblePARAMETERCDSCRCode mPLCode mSLtoffset0Group 0Code 0mPL0mSL0t01Code 1mPL1mSL12Code 2mPL2mSL23Code 3mPL3mSL34Group 1Code 4mPL4mSL4t15Code 5mPL5mSL56Code 6mPL6mSL67Code 7mPL7mSL7...124GroupCode 124mPL124mSL124t3131125Code 125mPL125mSL125126Code 126mPL126mSL126127Code 127mPL127mSL127
The above stored tables may involve, in the circuits provided, a need for a considerable amount of memory, above all in the perspective of multimode implementation of the future mobile terminals.